Electron sources having a needle-shaped electrode of tungsten single crystal and a coat layer of zirconium and oxygen formed thereon (hereinafter, referred to as ZrO/W electron sources) have been recently used to obtain a high-brightness electron beam having a life longer than hot cathodes (see Patent Documents 1 and 2).
In conventional ZrO/W electron sources, a coat layer of zirconium and oxygen (hereinafter, referred to as ZrO coat layer) is formed on a needle-shaped cathode of tungsten single crystal with its axis in the <100> direction.
When an electric field is applied to the cathode in that shape in vacuum apparatus, the electrostatic force as driving force causes mass migration toward the tip along the surface, forming a crystal face stabilized in surface energy on the cathode tip in the plane direction (100). The ZrO coat layer reduces the work function of the tungsten single crystal in the (100) plane from 4.5 eV to about 2.8 eV and makes only the fine crystal face formed in the tip region of the cathode in the (100) plane function as the electron emission region, and thus, such a device has an advantage that it can emit electron beam higher in brightness and yet it has longer life than conventional hot cathodes. It also has an advantage that it is more stabilized than cold-field-emission electron sources and easier to handle, as it operates even under milder vacuum.
In conventional ZrO/W electron sources, a needle-shaped tungsten cathode having the crystal face in the <100> direction and emitting electron beam is connected at a particular position to a tungsten filament formed on conductive terminals fixed to an electrical porcelain for example by welding. A supply source of zirconium and oxygen is formed locally on the cathode (for example, see FIG. 2). The surface of the cathode is covered with a ZrO coat layer.
Because conventional cathodes are used generally at a temperature of about 1800K, as the filament is heated under application of current, the ZrO coat layer on the cathode surface vaporizes. However, zirconium and oxygen are supplied to the cathode surface from the supply source continuously by diffusion, resulting in preservation of the ZrO coat layer.
In addition, the tip region of the cathodes used in conventional ZrO/W electron sources is placed and used in the region between a suppressor electrode and an extraction electrode (for example, see FIG. 3). A high voltage negative to the extraction electrode is applied to the cathode and a voltage negative to cathode of about hundreds of volts additionally to the suppressor electrode, thus suppressing emission of thermoelectrons from the filament.